1. Field
The present invention relates generally to data communication, and more specifically to techniques for polling for last finger on a channel in a digital communication system.
2. Background
In a wireless communication system, a modulated signal transmitted from a transmitter (e.g., a terminal) may reach a receiver (e.g., a base station) via a number of signal paths due to artifacts in the propagation environment (e.g., building, natural structures, and so on). A received signal at the receiver may thus include a number of multipath components, each of which corresponds to a signal instance received via a different signal path. Since the receiver may also receive signals from multiple transmitters, the received signal may thus include a number of multipath components for a number of transmitters.
For a digital communication system, a rake receiver is often used to process a number of multipath components in the received signal for a given “channel”. A channel may refer to, for example, a data transmission from a specific terminal. The rake receiver normally includes one or more searcher elements and a number of demodulation elements, which are often referred to as “searchers” and “fingers”, respectively. Each searcher may be operated to process the received signal to search for strong multipath components. Each finger may then be assigned to process a different multipath component of sufficient strength. By assigning a number of fingers to process a number of multipath components for the channel and combining demodulated symbols from all assigned fingers, more of the energy in the received signal may be collected for the channel and improved performance may be achieved.
Each multipath component is associated with a particular propagation delay and a particular time of arrival at the receiver, both of which are determined by the signal path. Each active finger is then associated with a respective time offset that is determined by the arrival time of the multipath component assigned to the finger. Each active finger would then process the received signal based on its time offset and provide demodulated symbols for its assigned multipath component. The demodulated symbols from each finger are “deskewed” (i.e., aligned in time) and accumulated with demodulated symbols from other fingers on the same channel to provide final demodulated symbols for the channel.
When multiple fingers are assigned to process multiple multipath components for a given channel, it is necessary to wait for the last finger to finish its processing before the final demodulated symbols for all assigned fingers can be processed by subsequent stages. The received signal can be buffered until the deskewed signals from all signal paths are received and then processed, or the demodulated symbols can be stored until the last finger is processed. In the later case, the last finger is normally the one assigned to process the latest arriving multipath component for the channel. Conventionally, the final demodulated symbols are buffered for a duration of time that is longer than the largest expected difference between the earliest and latest arriving multipath components (which is commonly referred to as the delay spread). By buffering the final demodulated symbols for this length of time, the last finger can be assured to have finished its processing, and the buffered final demodulated symbols may then be provided to subsequent processing stages.
This conventional mechanism for ensuring that the last finger has finished its processing is not efficient for several reasons. First, the buffering needs to cover the largest expected delay spread for the system, which may be long for some Code Division Multiple Access (CDMA) systems. Second, longer processing delay is incurred by having to wait for the largest expected delay spread to elapse before initiating subsequent processing on the final demodulated symbols.
There is therefore a need in the art for techniques to more quickly and efficiently determine when the last finger has finished its processing so that the final demodulated symbols may be expediently processed.